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Validation of Wind Tunnel Test and Cfd Techniques for Retro-Propulsion (Retpro): Overview on a Project Within the Future Launchers Preparatory Programme (Flpp)

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Validation of Wind Tunnel Test and Cfd Techniques for Retro-Propulsion (Retpro): Overview on a Project Within the Future Launchers Preparatory Programme (Flpp) VALIDATION OF WIND TUNNEL TEST AND CFD TECHNIQUES FOR RETRO-PROPULSION (RETPRO): OVERVIEW ON A PROJECT WITHIN THE FUTURE LAUNCHERS PREPARATORY PROGRAMME (FLPP) D. Kirchheck, A. Marwege, J. Klevanski, J. Riehmer, A. Gulhan¨ German Aerospace Center (DLR) Supersonic and Hypersonic Technologies Department Cologne, Germany S. Karl O. Gloth German Aerospace Center (DLR) enGits GmbH Spacecraft Department Todtnau, Germany Gottingen,¨ Germany ABSTRACT and landing (VTVL) spacecraft, assisted by retro-propulsion. Up to now, in Europe, knowledge and expertise in that field, The RETPRO project is a 2-years activity, led by the Ger- though constantly growing, is still limited. Systematic stud- man Aerospace Center (DLR) in the frame of ESA’s Future ies were conducted to compare concepts for possible future Launchers Preparatory Program (FLPP), to close the gap of European launchers [2, 3], and activities on detailed inves- knowledge on aerodynamics and aero-thermodynamics of tigations of system components of VTVL re-usable launch retro-propulsion assisted landings for future concepts in Eu- vehicles (RLV) recently started in the RETALT project [4, 5]. rope. The paper gives an overview on the goals, strategy, and Nevertheless, validated knowledge on the aerodynamic current status of the project, aiming for the validation of inno- and aerothermal characteristics of such vehicles is still lim- vative WTT and CFD tools for retro-propulsion applications. ited to a small amount of experimental and numerical inves- Index Terms— RETPRO, retro-propulsion, launcher tigations mostly on lower altitude VTVL trajectories, e. g. aero-thermodynamics, wind tunnel testing, CFD validation within the CALLISTO project [6, 7, 8]. Other studies were conducted to analyze the aerothermodynamics of a simplified generic Falcon 9 geometry during its re-entry and landing 1. INTRODUCTION burns, but up to now without comparable experimental data [9, 10]. Consequently, numerical and experimental tools are The Future Launchers Preparatory Programme (FLPP) aims at preparing the necessary components for future launch sys- not yet validated completely for the full range of a commer- tems to be able to serve Europe’s needs for a secured and cially relevant launcher trajectory, including high altitude autonomous capability of access to space in the future. Pri- control and propulsive maneuvers, as well as advanced aero- mary goal is to reduce costs as well as improve flexibility and dynamic control surfaces, like grid fins for example. availability to strengthen European launch operations and in- The project RETPRO, Validation of Wind Tunnel Test and dustry on the global market. Part of this endeavor, as stated CFD Techniques for Retro-propulsion, aims at closing this gap by the European Space Strategy [1], is to supply competence by preparing the necessary tools for a reliable design and sim- and technology to allow the re-usability of future and existing ulation of a complete ascent and descent trajectory of an inline spacecraft or components. proposed launcher configuration, which shall be based on cur- The paper at hand gives an overview on a 2-years ac- rent operational vehicles. For that purpose, the consortium, consisting of the Supersonic and Hypersonic Technologies tivity, funded by the European Space Agency (ESA), under Department and the Spacecraft Department of the DLR In- the framework of FLPP, dealing in particular with the mak- ing available of necessary tools for a detailed aerodynamic stitute for Aerodynamics and Flow Technology, and enGits, a and aerothermal design of future European vertical take-off supporting engineering company, providing efficient comput- ing resources and methods, follows a successive road-map: The RETPRO project, under which the disclosed research and author- ship was conducted, received funding by the European Space Agency (ESA) 1. Definition of a reference configuration, based on an op- within the Future Launchers Preparatory Program (FLPP). erational launch system, performing VTVL with retro- WP3: Computational Fluid Dynamics Flight / WTT CFD CFD for Numerical Rebuilding of WTT Extrapol. ale A B Model Design and Code-to-code Validation to Flight Numerical rebuilding / validation sc High fidelity 1 WP1: Reference Case WP2: Wind Tunnel Tests and Test Conditions Small Preparation of the Wind Tunnel Aerodynamic & Aerothermal Technical Work Packages Model Design Wind Tunnel Experiments flight to flight to 3 2 WP4: Joint Assessment of WTT/CFD and Flight Dynamics Simulation Flight Simulation, Extrapolation Extrapolation Continuous Updates of AEDB/ATDB Mission Analysis rules similarity to according 4 High fidelity AEDB High fidelity / fitted inviscid CFD WP5: Management Activities Fullscale D C Fig. 1. Work logic scheme Fig. 2. Strategy for AEDB/ATDB generation propulsion assisted descent and landing maneuvers. of an AEDB/ATDB for flight dynamics simulations. In indus- try, those tools are often low-fidelity or simplified computa- 2. Generation of high-quality aerodynamic and aerother- tional methods, derived from highly resolved Navier-Stokes mal test data from wind tunnel experiments through- computations. Usually, those simplifications are made in or- out the descent trajectory in the hypersonic, supersonic, der to maintain efficient design processes and short response and subsonic flight regimes with and without propul- time. Obviously, the simplification of sophisticated compu- sive jet. tational methods needs to be validated, but also the use of high-fidelity CFD codes in new fields of application needs to 3. Numerical rebuilding of wind tunnel experiments for be verified. validation of the numerical models in order to allow Therefore, a strategy of joint assessment of WTT and CFD a proper prediction of aerodynamic and aerothermal is developed (Fig. 2). In Step 3 of the before mentioned road loads. map, two high-fidelity CFD codes, the DLR TAU code and the DrNUM code by enGits GmbH (Sec. 6), are used to com- 4. Numerical extrapolation to full scale flight conditions pare with each other, as well as WTT results, representing in order to generate each, a sophisticated aerodynamic all relevant flight phases of a typical retro-propulsion assisted database and aerothermal database (AEDB/ATDB). vertical descent and landing trajectory. Prior to that, in Step 2, The different flight phases are experimentally simulated in 5. Simulation of the reference trajectories, enabling mis- three complementary WTT facilities at DLR Cologne (Sec. 5). sion performance analysis and final validation of the They are suitable for hypersonic, supersonic, and subsonic overall modeling approach. test conditions with additional cold and hot plume simulation capabilities using pressurized air or gaseous hydrogen and After summarizing the project strategy, the paper elab- oxygen (GH2/GO2) combustion in combination with the ex- orates on relevant currently operational launch systems and ternal ambient flow. Throughout the various flight phases, the thereon based derivatives for reference configurations and DLR TAU code is mainly used for rebuilding of WTT cases mission trajectories. The definition of a concept for the with hot plume simulation due to its capabilities in model- experimental tests, numerical computations, and their joint ing chemically reacting gas mixtures and serves as reference assessment for a reliable extrapolation to full scale flight con- for the calculations with the DrNUM code. The DrNUM code ditions is presented in line with the necessary experimental uses simplified hot gas modeling by an ideal gas mixture ap- and numerical tools for the conduction of wind tunnel tests proach and offers highly parallellized computing capabilities, (WTT), CFD computations, and flight dynamics simulations. using graphics processing units (GPU)s, which represents a The internal work logic scheme, depicting the described possible setup for an industrialized design process as men- road-map is shown in Fig. 1. tioned above. Based on a successful validation, both CFD codes are used 2. PROJECT STRATEGY in Step 4 to extrapolate the WTT results to a realistic flight configuration by performing full scale computations. The re- The main goal of this project focuses on the validation of sults of the full scale computations are gathered in order to tools, necessary for future design considerations in the field of sufficiently populate an AEDB/ATDB for a final 3-DoF or supersonic and subsonic retro-propulsion and the generation 6-DoF flight simulation, which is performed using the DLR in-house tool FDS (Flight Dynamics Simulator) (Sec. 7) in Step 5. Since the first step of the project’s road map is to define reference configurations and missions based on operational launch systems, the derived flight simulations can finally be compared to original flight data of the reference cases in order to close the validation cycle. 3. SELECTION OF THE REFERENCE LAUNCHER AND MISSIONS 3.1. State of the Art (a) (b) (c) (d) There are a couple of concepts for reusable launch vehicle Fig. 3. State of the art in vertical take-off and landing reusable (RLV), existing in the United States, that have already been launch vehicle (VTVL RLV) in the United States: (a) DC-X, flown or tested to different degrees. Hence, they involve dif- 1993; (b) New Shepard, 2016; (c) Falcon 9, 2015; (d) Falcon ferent technology readiness levels (TRL), as well as different Heavy, 2019 ranges of mission envelopes and are therefore more or less sufficient as the project’s reference configuration. The Delta Clipper Experimental (DC-X) vehicle (later NASA DC-XA), Mission Flight Date Type Orbit Payload a conceptual single-stage-to-orbit (SSTO) vehicle was tested NROL-76 33 01/05/2017 RTLS LEO 2,800 kg in the 1990s up to an altitude of 3.1 kilometer (Fig. 3, a). Blue SES-11 43 11/10/2017 DRL GTO 5,200 kg Origin’s New Shepard, a sub-orbital rocket for technology demonstration and space tourism, which reached an altitude Table 1. Falcon 9 reference missions of slightly above 100 kilometer, is sporadically operated since 2015 (Fig. 3, b). But only SpaceX’s Falcon rockets, i. e. the Falcon 9 and Falcon Heavy, are currently in regular commer- cial operation since 2013 and 2019 respectively (Fig.
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